Wavelength-Division Multiplexing (WDM) systems are known in which optical signals, each modulated to carry a different information stream and each having a different wavelength, are combined into a WDM signal and transmitted on an optical communication path including one or more segments of optical fiber, for example. Such WDM signals have greater capacity than systems in which a single modulated optical signal having a single wavelength is transmitted. Further information carrying capacity can be achieved by transmitting different polarization components or portions of each signal that are modulated independently of each other to carry separate data streams. For example, one component of the optical signal may have a transverse electric (TE) polarization and another component of the optical signal may have a transverse magnetic (TM) polarization that is oriented orthogonal to the TE polarization. By separately modulating and combining (or multiplexing) TE and TM polarized optical components at each wavelength, the information carrying capacity can be doubled compared to a WDM system that transmits optical signals having a single polarization at each wavelength. WDM systems in which each optical signal has combined TE and TM optical components are often referred to as Polarization Multiplexed (PM) WDM systems.
Typically, optical signals transmitted in a PM WDM system are attenuated as the optical signals propagate along the optical fiber. In order to compensate or offset such attenuation, the power level of the optical signals may be boosted or increased by optical amplifiers provided at periodic locations along the optical communication path. Typically, the gain imparted to each optical signal, as well as each polarization component (TE and TM) of each optical signal, should preferably be the same. Otherwise, those optical signals and polarization components experiencing increased gain will be amplified more at each amplifier site than those experiencing less gain. The power of such high gain optical signals and polarization components will increase at each successive amplifier, while the power associated with the low gain optical signals and polarization components will decrease at each amplifier site. After propagating through several amplifiers, most of the amplification will be imparted to the high gain optical signals and polarization components, while the low gain optical signals and polarization components will be attenuated and indistinguishable from a background noise level when supplied to an optical receiver.
Although capacity gains have been realized with PM WDM systems, further capacity increases may be required. Such capacity increases may be realized by providing further polarization multiplexed optical signals at additional wavelengths. There is a need, however, to adequately or uniformly amplify each of the optical signals in such high capacity systems. Further, there is a need to provide such amplification with an amplifier having a compact design.